The present invention relates to a process for separating from an aqueous mixture one or more organic substances containing at least one positively charged and/or chargeable nitrogenous group by means of extraction via at least one porous membrane.
Numerous techniques for the separation of organic compounds from aqueous solutions are known, such as separation via ion exchange resins, chromatography processes, adsorption, filtration, evaporation, reverse osmosis, electrodialysis, etc.
In this relation the recovery of amino acids in particular has gained in economic interest in recent years, in the first place in connection with the foodstuffs and beverages industries. Separation of the required amino acids from mixtures is effected for instance in particular via ion exchangers or for instance by means of reactive extraction. Where purification of especially culture broths is concerned, limitations are encountered with these processes however. When culture liquids are treated by means of ion exchangers it is a drawback that the mixture to be treated requires an extensive pretreatment.
For the purification of in particular L-phenylalanine extraction processes involving several liquid membranes have been described (Thien, M. P. et al., Biotechnol Bioeng, 1998, 32: 604-615). In the widest sense this relates to oil-in-water emulsions with formation of organic, for instance lipids-containing, membrane vesicles, through which the actual extraction then takes place. However, at high product concentrations such liquid-liquid extraction processes are subject to limitations due to considerable swelling of the membrane vesicles. In addition, the stability of the membrane vesicles is pressure sensitive.
Dispersion-free extraction processes via hollow fibres for separation of L-phenylalanine are already known from Escalante H. et al. (1998, Separation Science and Technology, 33(1): 119-139). The extraction agent used in this case is a mixture of quaternary ammonium salts, isodecanol and kerosine, which, it should be noted, exhibits a strong selectivity towards anions. The drawback of this process is that the extraction of L-phenylalanine requires a titration of the medium at a pH value of 10.5. The re-extraction from the organic phase requires another titration at a pH value of 0.5. In addition, because of the toxicity of the extraction agent, in particular the quaternary ammonium salts, which are also used as disinfectants, direct recycling of the medium into the production process is not possible. An additional drawback of this process lies in the circumstance that separation of the L-phenylalanine cannot be integrated with the fermentation process, so that it is not possible to prevent a potential product inhibition.
The extraction of amino acids via a cation-selective extraction agent consisting of di-2-ethylhexylphosphoric acid dissolved in n-decanol has been described by Teramoto M. et al (1991, J. Membr. Sci. However, this involves a liquid-membrane system, presenting the drawbacks mentioned in the foregoing, especially in respect of the low stability of the liquid membrane vesicles. A further drawback is that the components of the extraction agent used are toxic to biological production systems, e.g. fermentation systems, which means that the scope of application of this process is restricted to batch extraction. Also in this case therefore it is not possible to make use of an integrated separation system in order to prevent a potential product inhibition.
Wieczorek S. et al. (1998, Bioprocess Engineering, 18: 75-77) describes the use of a mixture of tridodecylamine, kerosine and octanol as extraction agent. This process is not aimed at recovery of amino acids, as in the case of the systems referred to above, but at separation of citric acid, a nitrogen-free substance, from the culture broth of the fungus Aspergillus niger by means of an anion-selective carrier. Also in the case of this system, the high toxicity of the extraction agent is a drawback. That is why returning of the medium to the production process requires the introduction of additional, costly process or, more specifically, purification steps, as a consequence of which, moreover, only partial recycling of the medium can be achieved.
The aim of the present invention therefore is to provide a process for separating from an aqueous mixture one or more organic substances containing at least one positively charged and/or chargeable nitrogenous group by means of extraction via at least one porous membrane, which process does not present the above-mentioned drawbacks.
This aim is accomplished due to the use of an extraction agent which contains at least partially relatively long-chain organic compounds and at least one liquid cation exchanger, and of a membrane that is wettable by either the aqueous mixture or by the extraction agent.
The liquid cation exchanger serves as a carrier. According to the invention the organic substances are preferably re-extracted from the extraction agent into an aqueous phase.
The relatively long-chain compounds used preferably are compounds which are poorly miscible with or poorly soluble in water and are liquid at temperatures between 10 and 60xc2x0 C., preferably between 20 and 40xc2x0 C. Compounds with 6 to 20 C. atoms are preferred according to the invention; particular preference is given to compounds with 12 to 18 C. atoms. Such compounds may be branched, non-branched, saturated, non-saturated or partially aromatic organic compounds. Examples of relatively long-chain organic compounds according to the invention are alkanes, alkenes or fatty acid esters or mixtures of several of these compounds. These compounds serve as solvents in the process according to the invention.
Alkanes to be used are for instance hexane, cyclohexane, decane, ethyl decane, dodecane or mixtures thereof. Kerosine is particularly preferable. Suitable alkenes are for instance hexene, nonene, decene, dodecene or mixtures thereof. Suitable fatty acid esters are in particular alkyl stearates with alkyl groups having more than 2 C. atoms. Examples of fatty acid esters are ethyl stearate, butyl stearate, isopropyl stearate, ethyl palmitate and butyl linoleate. Particularly preferable are kerosine and butyl stearate. It is also possible to apply two or more of said organic compounds in the form of a mixture.
The liquid cation exchangers employed are preferably esters of inorganic acids and organic groups, which are preferably branched. The inorganic acids preferably are phosphoric acids, phosphorous acid, sulphuric acid and sulphurous acids. Phosphoric acid is preferred in particular. The organic residue groups applied according to the invention preferably are branched and/or non-branched alkyl or alkenyl groups with at least 4 C. atoms, preferably 4 to 20 C. atoms. The preferred liquid cation exchangers include di-2-ethylhexyl phosphoric acid esters, mono-2-ethylhexyl phosphoric acid esters, dinonylnaphthalene sulphonic acid esters or mixtures thereof. Preferred according to the invention is the mixture of di-2-ethylhexyl phosphoric acid ester and mono-2-ethylhexyl phosphoric acid ester. The mono-2-ethylhexyl phosphoric acid ester content of this mixture is preferably over 40 wt. %, more in particular over 80 wt. %, relative to the total amount of liquid cation exchanger.
Said liquid cation exchangers are preferably present in the extraction agent in an amount of 2 to 25 wt. %, relative to the amount of relatively long-chain organic compounds; particularly preferred are amounts of 5 to 20 wt. % of liquid cation exchanger and most preferred are amounts of 8 to 15 wt. % of liquid cation exchanger. According to the invention the extraction agent may contain other substances, including state-of-the-art extraction agents, besides the compounds mentioned here.
For the process according to the invention it is preferred to make use of a porous membrane which is wettable by either the aqueous mixture or by the extraction agent and has a pore density of 5 to 95%, more in particular a pore density of xe2x89xa730%, most preferably a pore density of xe2x89xa740%. The state-of-the-art porous membranes which are wettable by either the aqueous mixture or by the extraction agent can in principle be used for the process according to the invention. Membranes with a maximum pore density are preferred, particular preference being given to for instance hollow-fibre contactors.
Preferably, porous membranes with a pore size of xe2x89xa62 xcexcm are used for the process according to the invention, more in particular a pore size of xe2x89xa61 xcexcm, more preferably a pore size of xe2x89xa60.5 xcexcm, most preferably a pore size of xe2x89xa60.05 xcexcm.
The process according to the invention is suitable in particular for the extraction of organic substances which contain at least one positively charged and/or chargeable nitrogenous group. Most preferably the process according to the invention can be used for the extraction of organic substances belonging to the group of aliphatic and/or aromatic amino acids and/or lactams, the salts, derivatives or di- or oligopeptides thereof or mixtures of these compounds.
The substances to be extracted include for instance L-amino acids or D-amino acids. In principle natural as well as non-natural amino acids can be extracted, such as all D- and L-forms of essential amino acids. Examples of extractable amino acids are L-phenylalanine, D-phenylalanine, L-tryptophane, D-tryptophane, L-tyrosine, D-tyrosine, D-p-hydroxyphenylglycine, D-phenylglycine, Di-hydroxyphenylalanine. Lactams can also be extracted by means of the process according to the invention, for instance xcex2-lactams, caprolactam, penicillin G. Further, the process according to the invention can be used for the extraction of peptides, in particular di- or oligopeptides, for instance L-aspartyl-L-phenylalanine as a precursor molecule for the preparation of aspartame. Amino alcohols, for instance 1S, 2R-cis-(-)-aminoindanol, can also be extracted. Extraction according to the invention can also be applied for the recovery of amines or amides.
A preferred area of application of the process according to the invention is extraction from fermentation solutions, effluent flows and/or aqueous mixtures from chemical synthesis and/or degradation processes. In particular, the process according to the invention can be integrated into fermentation processes. The fermentation processes can be of an aerobic or an anaerobic nature and can be operated as batch, semi-continuous or continuous processes.
The invention further relates to a process which comprises the following steps:
a) the aqueous mixture is drawn from a reservoir,
b) led across a first porous membrane which is wettable by either t the aqueous mixture or by an extraction agent which contains at least partially relatively long-chain organic compounds and at least one liquid cation exchanger,
c) extracted with the extraction agent,
d) the aqueous retentate is returned to the reservoir,
e) the extracted organic substances are led across a second porous membrane which is wettable by either the aqueous mixture or by the extraction agent and
f) there re-extracted into an aqueous phase.
The process is preferably set up in such a way that in step d) the aqueous retentate is returned completely to the reservoir. It is also preferred for the pressure difference between the aqueous mixture and the extraction agent to lie between 0.1 and 10 bar, more in particular between 0.5 and 5 bar, most preferably between 2 and 3 bar.
In a particularly preferred embodiment of the process according to the invention the re-extraction can be effected with simultaneous concentration augmentation of the organic substances. Particular preference is given to re-extraction with simultaneous concentration augmentation of the organic substances.
A particularly preferred application area of the present invention is the extraction of substances from fermentation solutions. The fermentation processes can be of an aerobic or an anaerobic nature and can be operated as batch, semi-continuous or continuous processes. The process according to the invention is preferably set up in such a way that the extraction takes place continuously and simultaneously with a reaction that is proceeding in reservoir 1, for instance a fermentation reaction, i.e. in such a way that the extraction is integrated. As a special embodiment of the process according to the invention, the re-extraction of the organic substances into the aqueous phase can also take place as a process that is integrated relative to a reaction that proceeds in reservoir 1. The object of the invention is not restricted to such processes, however.